Was this poor etiquette in the endgame?

I'm a fairly low skilled player (but working on it), so I'm aware that the game was fairly sloppy and I made a bunch of mistakes early on, but I wanted to focus on etiquette. After finishing this game, I received a long message from the other player cursing me out and calling me a childish troll for taking it to time out instead of resigning. In my opinion, time is as much factor in many games as anything else. Because of that, I think it's fair for me to count on the opponent making mistakes and being unable to win with their time remaining.

On the other hand, I do understand that it could be annoying to do this, but due to a lack of experience I'm not really sure what the proper protocol is. Thanks in advance for any guidance!

In a blitz game? No never resign in blitz, he was just mad he didn't find the right moves to win faster.

Don't ever give a free win, make them earn it. Unless all hope is lost and you know your opponent will win with plenty of spare time, fight for the draw. Its totally worth doing. If they don't want to be timed out, they will play with increment

There is no reason why he should be given a full point for that: he didn't have enough time on the clock to win the game, period.

Another thing would be if you still had a pawn or so and would win on time. If it was blitz or bullet, that's a sure win - in these time controls, time is as important as the position.

In standard and rapid time controls ("classical" here) FIDE rules state (roughly!) that one can claim a draw if he has less than two minutes on the clock and the opponent cannot win by normal means; this is subject to the arbiter's decision.

Now, there are no arbiters on lichess and this rule clearly does not apply here. On the other hand, I would consider it bad manners to win on time in a completely lost position in "classical" time controls. I would expect one to accept a draw offer in such a case.

Again, it is ok to force draw by timeout in a draw position. And it is even worse to insult other people: I'd suggest you to report it.

At move 51/63 he had 2.9 seconds on the clock, and the 1300 level there will be many players won't even be able to mate with a K+Q vs K in that timeframe with nothing else on the board.

It was definitely worth playing for the flag, which you did and succeeded. He chose to berserk a 3 minute game, and you punished him for it.

He did berserk from the start, if he finds himself then unable to win with the reduced time, he can only blame himself.

In my opinion a good protocol is: chess should be fun! It seems at some point your opponent wasn't having fun.

Dunno why he didn't play 20. Bf6 or 20. Bh6, but instead traded into a dull and challenging endgame.

When you seek/accept a challenge you've acknowledged that losing on time is a possibility. I can almost guarantee that even if you would of won easily against them they would of still been talking curse words to you. Do not feel bad because you got a sore drawing opponent. He was just upset at his poor time management skills.

Complaining about getting flagged would be ridiculous even if he hadn't berserked, which he did.

Never feel bad about winning on time, or even trying to win on time. Time management is just as much a part of the game as moving the pieces is. Just laugh at the person to yourself and move on. Your opponent managed his time poorly and threw a tamper tantrum when it cost him, it happens all the time.